1. Field of the Invention
The present invention relates to a telecentric illumination system. More particularly, the present invention is directed to improvements in such illumination system, and in which the illuminated area can be changed.
2. Description of the Prior Art
Telecentric illumination has been used, for example, for the illumination system in a measuring projector or the like. The purpose for which the telecentric illumination is employed is to minimize the effect of focusing error on measurement. For projection examination it is desired to obtain a clear and bright projected image irrespective of magnification used. To meet the desire, there have been proposed various variable power illumination systems which enable changing the illuminated area according to the change of magnificaton of the objective lens. FIG. 1 illustrates an example of the simple illumination area changeover system according to the prior art. FIG. 1A and 1B show two different states of illumination by use of an illumination system comprising a positive and a negative lens. The state of illumination shown in FIG. 1A is for a lower magnification and that shown in FIG. 1B is for a higher magnification. At the higher magnification, the illuminated area becomes narrower than that at the lower magnification. As will be described hereinafter, the known illumination system has the drawback that when the magnification is changed from a lower magnification as shown in FIG. 1A to a higher magnification as shown in FIG. 1B, the marginal rays become unusable for illuminating the narrower area and therefore the efficiency of the illumination often drops down. This will be described in detail with reference to FIGS. 1A and 1B.
In FIGS. 1A and 1B, the sample surface to be illuminated is indicated by O. Dotted lines indicate oblique rays.
In the case of FIG. 1A for a lower magnification, the composite focal length is relatively long, and therefore the beam diameter is relatively large. The angle which the oblique rays form with the optical axis of the illumination system is small. Under this state of illumination, the quantity of marginal light incident on the sample surface (O) is sufficient.
In the case of FIG. 1B for a higher magnification, however, the composite focal length is relatively short and therefore the beam diameter is small. Consequently, the angle which the oblique rays form with the optical axis becomes larger as compared with FIG. 1A. A large part of the oblique rays misses the sample surface (O). The efficiency of illumination decreases accordingly. This is the most important drawback of the shown illumination system according to the prior art.
As a solution to the above problem of poor illumination at high magnification, condensing illumination has been employed in many apparatus, but only for want of a better technique. However, when condensing illumination is employed in an illumination system, the illumination system can no longer maintain the conditions for telecentric illumination. Such illumination system can not be satisfactorily used as an illumination system for a measuring instrument or other similar apparatus for which extremely high accuracy is required.
The known illumination system also has a problem in measuring a cylindrical object. When a cylindrical object is illuminated by the above known illumination system, there is produced a partial shadow due to the effect of the illumination aperture. In this case, the geometric boundary of the measured object and the optical boundary can not exactly coincide with each other, so that a measuring error is caused. To minimize such error, the illumination must be done through an aperture properly adjusted relative to the diameter of the measured object in accordance with the well-known Gunther's formula. "Die Microskopische Abbildung von Zylindern and Gewinden" appearing on pages 315-321 of Zeitschrift fur Instrumentekund, August 1939. However, in the conventional changeover type simple illumination systems that may be employed in a measuring projector and other similar measuring apparatus, it is generally difficult to incorporate into the system a telecentric stop (aperture stop) for controlling the aperture.